At a working station, it is generally desirable to have a certain level of light (i.e. light intensity), and in many countries there are even health regulations specifying a minimum level of light for the workers. This level may depend on the type of work being performed. To provide such light level under all circumstances, it is necessary to arrange light sources (lamps) capable of providing said light level. However, during daytime, sunlight may enter the room in question, and may contribute to the light level at the working station. If the lamps would be operating at their nominal light output, the light level would be higher than necessary. Thus, energy can be saved if the light output of the lamps is reduced with increasing daylight level, such that the total light level remains substantially constant.
For this purpose, systems have been developed comprising dimmable lamps, and a lamp controller for controlling the dim level of the lamps on the basis of the daylight level. The daylight level is measured by one or more daylight sensors, which provide(s) a measuring signal to the controller.
In new systems to be installed, such daylight sensor(s) may be coupled to the controller by wires, but greater flexibility y is obtained if wireless sensors are used. Further, in existing buildings with existing illumination systems, it is easier to use wireless sensors as compared to the necessity of arranging wirings. A wireless sensor provides its measuring signal to the controller by a suitable wireless communication transmission, such as for instance ZIGBEE.
A wireless sensor will be powered from a dedicated power source such as a battery, possibly recharged by a photovoltaic converter using captured daylight if the higher system costs are acceptable. With such dedicated power source, it is desirable to keep power consumption low in order to have a long service time, i.e. life time of the battery or the like. On the other hand, the wireless transmission consumes relatively much energy. Thus, it has already been proposed that the sensor does not communicate to the controller constantly, but intermittedly. For instance, if the sensor only transmits a measuring signal once per second, and the transmission duration is 20 ms per transmission, the energy consumption is roughly reduced by a factor 50.
US patent application 2008/0007394 describes an illumination system where each light source is provided with a sensor for sensing the light level close to the light source. Here, the light sensor is not wireless, is not powered from a battery, and transmission of measuring signals is continuous, reduction of power consumption clearly not being an issue.
U.S. Pat. No. 6,340,864 describes an illumination system with a wireless sensor, which transmits its data periodically in small bursts, for instance once every second.